It has been proposed to utilize atmospheric fluidized bed combustors operating in series to provide a working fluid for closed-cycle gas turbine power generation systems at a temperature of 1700.degree. F. or higher. A working fluid, e.g., air, at temperatures of about 1500.degree. F. can be obtained by inserting conventional metal heat exchange surfaces in a fluidized bed operating at 1600.degree. to 1700.degree. F. Heating the working fluid to a final temperature above about 1700.degree. F. in fluidized beds which operate at 1850.degree. to 2000.degree. F. requires the use of ceramic surfaces (which lack ductility), in the fluidized bed.
A need therefore exists for an effective indirect heat exchange system for use in fluidized bed combustion zones.